1. Field of the Invention
The invention relates to a continuous process for producing mixtures of amorphous polyolefins having a broad molar mass distribution and a uniform glass transition temperature.
2. Description of Related Art
To produce a bimodal or multimodal mixture of amorphous polyolefins, two or more polyolefins having different molar masses have to be mixed and homogenized. In the case of a small difference in the molar masses of the starting components and consequently a small difference in the melt viscosities, mixing can be carried out in the melt. This occurs in the case of extrusion. However, above a certain difference in the melt viscosities, homogeneous mixing of amorphous polyolefins in the melt can no longer be carried out. According to Karam, Bellinger, Ind. A. Chem. Eng. Fund 7(1968) 4, 571–581, this limit is reached when the viscosity ratio of secondary component to main component of the mixture is less than 0.005 and greater than 4. Accordingly, narrow limits are imposed on the effective mixing of a relatively high molecular weight polymer into a low molecular weight matrix via the melt. Melt mixing can only be carried out by means of a number of melt mixers connected in series. However, such a process has high capital costs and process costs and its economics are therefore poor.
EP-A-0 843 223 discloses a bimodal toner. The mixture is produced batchwise. The relatively high molecular weight component has a molecular weight (Mw) of 70,000 g/mol, a viscosity number (VN) of 80 ml/g and a glass transition temperature above 70° C. The toner is produced by mixing the starting components in the melt.
In the case of large differences in the molar masses of the blend components, the melt viscosities differ so much that the production of a homogeneous blend via the melt is possible only with great difficulty.
WO 98/29783 discloses a tower having a broad molar mass distribution (bimodal, multimodal, broad distribution without separate peaks). The preparation of the base material and mixing were carried out batchwise. The relatively high molecular weight component had an Mw of 100 000 g/l and VN of 130 ml/g, therefore somewhat higher than EP-A-0 843 223. In the case of large differences in the molar mass of the blend components, the melt viscosities differ so greatly the production of a homogenous blend via the melt is extremely difficult.
EP-A-0 128 045 discloses a process for preparing crystalline polyolefins. The catalyst system for the polymerization of ethylene to form polyethylene comprises two different metallocenes. The process of homogeneous catalysis and the resulting polyethylene having a polydispersity of from 2 to 50 are likewise described. In contrast, corresponding catalyst systems for preparing amorphous cycloolefin polymers are extremely difficult to find. Firstly, they have to catalyze the reaction highly specifically without forming light-scattering by-products which reduce the transparency of the material, and, secondly, these catalysts should display the same copolymerization diagram so that under identical reaction conditions a plastic having only one macroscopically observable glass transition temperature is formed.
WO 96/18662 discloses a process for preparing polyethylene in which the first stage is carried out in a low-boiling hydrocarbon in a loop reactor, the second stage is likewise carried out in a solvent in a loop reactor and the third stage is carried out in the gas phase. In each stage, further catalyst, cocatalyst, ethylene or hydrogen can be added. The high molecular weight component is prepared in the first stage. In the gas-phase reactor, a C4–C8-α-olefin can also be added as comonomer. This process cannot be applied to the preparation of cycloolefin copolymers since gas-phase reactors are unsuitable for liquid comonomers. Furthermore, the catalysts should catalyze the reaction highly specifically without light-scattering by-products being formed.